Extremely narrow bandwidth optical filters can be based on birefringent elements and the interference of polarized light. In addition to high spectral resolution, such filters provide a wide field of view and tunability. The two basic designs are the Lyot-Ohman and the Solc filters (see, for example, A. Yariv and P. Yeh, Optical Waves in Crystals, Wiley, N.Y., 1984, 133-143). Rapidly tunable birefringent filters of the Lyot design have been demonstrated (Johnson et al., U.S. Pat. No. 5,132,826. Lyot filters consist of multiple birefringent elements with retardation which increases geometrically in powers of two for successive stages. The stages are separated by parallel polarizers. Throughput is reduced by the multiple polarizers and, because the birefringent plates have different retardations, the Lyot filter does not lend itself to a multipass configuration.
A birefringent filter which overcomes these limitations is the multiple retarder Solc filter. A Solc filter requires input and exit polarizers, and a series of identically thick wave plates which are half-wave or full-wave retarders at the design wavelength. For the folded Solc filter configuration, these are half-wave plates oriented at alternating rocking angles +.rho., -.rho. along the direction of propagation. At the design wavelength, i.e. the wavelength at which the retarders are half-wave plates, the polarization is reflected about the optic axis of each wave plate. The term "reflection of polarization" about an axis is used in the art to refer to a rotation of the polarization by 2.theta., from an orientation of .theta. with respect to the axis to an orientation of .theta.. By choosing the rocking angle to be .rho.=.pi./4N, where N is the number of wave plates, light at the design wavelength is rotated by 90.degree. while light at other wavelengths does not experience a 90.degree. rotation. Thus an exit polarizer at 90.degree. to the input polarization transmits a narrow spectral peak centered at the design wavelength. The transmission spectrum contains a series of peaks for every wavelength which satisfies the half-wave condition of the retarders, and additional blocking filters can be employed to select a specific peak. The spacing between these peaks is the free spectral range of the Solc filter. For the fan Solc filter configuration, the wave plates are full-wave rather than half-wave retarders at the design wavelength. They are oriented at .rho., 3.rho., 5.rho., . . . and the polarizers are parallel to one another. In this configuration light at the design wavelength is unaltered by the waveplates while the polarization of light at other wavelengths is rotated off the axis of the polarizers. When .rho.=.pi./4N the maximum discrimination is achieved.
A Solc filter can be implemented by multiple passes through a Fabry-Perot cavity as first proposed by J. Katzenstein (J. Opt. Soc. Am., 58, 1348, 1968). This greatly reduces the required number of birefringent elements and, since the series of wave plates is replaced by a single plate, the requirements of uniform plate retardance and orientation are automatically satisfied. The filter can be tuned by varying the retardation of the center wave plate, for example by electro-optic means, to select the wavelength at which it provides a half or full wave of retardation.
The filter can be constructed with three intracavity birefringent plates: a central retarder with quarter-wave plates on either side (J. H. Williamson, J. Opt. Soc. Am., 61, 767 (1971)). The central retarder is oriented at the rocking angle .rho.. The first quarter-wave plate is oriented with optic axis parallel to the input polarizer and the second is parallel to the exit polarizer. The quarter-wave plates function as passive rotators. That is, light passing through a quarter-wave plate, reflected from a mirror and passing again through the quarter-wave plate has traveled an optical path equivalent to a single pass through a half-wave plate and the polarization is rotated about the wave plate optic axis. This rotation allows the optical field to see the central retarder as a series of wave plates at +.rho. and -.rho. as required for a folded Solc filter, and at .rho., 3.rho., 5.rho. . . . for the fan filter.
According to mathematical analyses by Weis and Gaylord (R. S. Weis and T. K. Gaylord, J. Opt. Soc. Am. A, 4, 1720, 1987) it should be possible to use polarization interference and Fabry-Perot interference effects synergistically in a multipass Solc device to provide a high resolution tunable filter. Using matrix methods for analysis they predict that the transmission spectrum is the product of the Fabry-Perot fringes and the Solc filter transmission function. Thus the peaks which satisfy the Solc polarization-interference condition form an envelope for the Fabry-Perot fringe intensities. If the half-width of the Solc envelope is less than the free spectral range (FSR) of the cavity resonances, then only one Fabry-Perot peak falls within the Solc envelope. Thus the FSR of the filter is the Solc envelope peak spacing rather than the Fabry-Perot fringe spacing and the effective finesse of the filter can be extremely large. A variable isotropic spacer can be employed to tune the optical path length and thereby tune the cavity resonances.
The wavelengths of the polarization-interference transmission peaks can be tuned by changing the retardation of the central retarder. This can be implemented by various methods. Temperature and mechanical tuning techniques are inherently slow. Electro-optic tuning of the birefringence with applied electric field is more rapid. Weis and Gaylord modeled the electro-optically tuned Fabry-Perot Solc filter using LiNbO.sub.3 for the spacer layer and either LiNbO.sub.3, KNbO.sub.3, or Hg.sub.2 Cl.sub.2 for the central retarder. They predict a filter linewidth of 0.01 nm, which is two orders of magnitude narrower than available isotropic thin-film narrow-band filters. The limitation in these materials comes in the tuning range. For full tuning over the FSR of the Solc envelope the central retarder must be tunable over 2.pi. in retardation. The tuning range of the birefringence depends on the material properties, the applied electric field, and the plate thickness. Even with a material breakdown limited maximum applicable electric field of 10.sup.7 V/m, plate thicknesses on the order of hundreds of microns are required for the electro-optic materials employed by Weis and Gaylord to achieve a full cycle of retardation change. This reduces the FSR of the Fabry-Perot fringes and limits the ability to design the cavity to select free spectral range, finesse, and out-of-band rejection according to the requirements of the application.
These constraints can be overcome by the use of liquid crystal variable retarders, as described in the present invention. Liquid crystal molecules are long organic structures characterized by a molecular director which re-orients in response to an applied electric field. An optical field interacting with a liquid crystal cell experiences varying retardation when the motion of the molecular director is out of the plane of the optical field. Achieving variable retardation is a function of the alignment of the crystal within the plates which contain the liquid, the placement of electrodes parallel or lateral to these plates, and the direction of light propagation through the ends or sides of the cell. Liquid crystal cells can be tuned over 2.pi. in retardation with modest voltages and film thicknesses.
Nematic liquid crystal cells can provide analog retardation changes with the application of an electric field. Typical tuning speeds for nematic materials are 10-100 msec. Chiral smectic liquid crystals (CSLC's) are characterized by rotation time constants as short as 100 nsec, six orders of magnitude faster than the nematic materials. Homeotropically aligned CSLC's provide retardation changes with application of an electric field parallel to the containing plate. However this requires lateral electrodes, which may limit the device aperture. Surface stabilized planar aligned CSLC cells are constructed with transparent electrodes parallel to the containing plate and thus can have a large clear aperture. However, because the molecular director rotates in the plane of the containing glass plates, and therefore the plane of the optical field, there is no change in retardation with the application of an electric field. Consequently, these wave plates can be modeled as mechanically rotatable fixed-retardation wave plates. Variable retardation can be achieved by the use of three wave plates: a rotatable CSLC half-wave retarder and two quarter-wave plates, as described by Biernacki et al. (Proc. SPIE 1455, 167, 1991) and by Sharp et al. (U.S. patent application 07/792,284 filed Nov. 14, 1991). For a liquid crystal with a tilt range of at least .pi./2, this quarter-half-quarter compensator can achieve variable retardation between 0 and 2.pi.. In contrast to single element birefringent retarders, the polarization of linearly polarized light is preserved.
Liquid crystal cells have been utilized in a number of optical filters. A Fabry-Perot etalon filter with a nematic liquid crystal cell changes the optical path length and thus tunes the Fabry-Perot fringe wavelengths, but lacks the Solc polarization-interference to select among the fringes (M. W. Maeda et al., IEEE Photonics Technology Lett. 2, 820, 1990) and A. Miller et al. in U.S. Pat. No. 4,790,634 and J. S. Patel in U.S. Pat. No. 5,068,749). Tunable birefringent Lyot filters have been realized with nematic liquid crystal cells (W. I. Kaye in U.S. Pat. No. 4,394,069) and with chiral smectic liquid crystals (Johnson, op. cit.). However, because the elements require a progression of retardations, these have not been implemented in a multipass configuration.